A zone is heated space in which a temperature responsive device, such as a thermostat, controls the operation of a common boiler or heat source for all of the zones. A valve or a separate pump sends hot water or steam to the particular heat zone measuring it. The control for each zone will turn on the common boiler or heat source. But, the hot water or steam circulation takes place only throughout the selected zone. A zone typically includes more than one heat radiation element. All elements of a zone are heated simultaneously, even if parts of the particular zone are not occupied or used by persons except at special times of the day or night. Thus, smaller size, and correspondingly more numerous zones are preferred.
Running separate piping systems or ducts for each zone and providing a separate control element for each zone is expensive and space consuming. Hot water systems are usually built with no more than two or three zones in the largest homes and steam systems usually have only one zone for an entire home.
Converting an existing single zone system to a multi-zone system or upgrading of two or three zone systems to a higher number of zones is totally out of the question economically or in terms of the physical remodeling which is necessary to accomplish this in most homes and small buildings.
Within a single zone, individual rooms may be too hot or too cold. Some are occupied at one part of the day and others at other parts of the day or night. Some adjustment for a too hot room is achieved by only partially opening the steam inlet valve or by some control of the steam outlet vent valve, or by choking off the flow of hot water in the case of hot water systems. This does not provide for efficient distribution of heat among all heat radiating elements in a zone. All of the heat radiating elements are being heated by the steam or hot water at the same time. If the largest radiator is used in a particular room and the room is still too cold, in a single zone system, all the other rooms are heated to a greater extent to get that particular room up to temperature.
At night only sleeping quarters require an elevated temperature while the rest of the house can be permitted to become much cooler. By day the reverse is true. However, with a single zone system, living rooms are kept up to temperature if the bedroom is to be kept comfortable.
To supply all the rooms in a zone with steam at the same time, the steam boiler or hot water furnace must be sized appropriately. This means that at start-up, for instance, in the morning, a much larger volume of water must be heated to get the system up to day temperature than would be necessary if steam or hot water were to go in only selected rooms and in a given sequence following a family living pattern through the day.
Schools and offices which use thermostats in every room effect closer zone control only by opening or closing valves leading to radiators in that room. This makes it necessary to have the steam or hot water in the pipes outside the room. The zone system merely channels the heating fluid in the form of steam or water to the proper radiator. The steam or hot water furnace is controlled by a sensor in a single area or is controlled manually by a custodian who tries to maintain the building at an average temperature level.
With this type of cycling, a room close to the boiler receives all the steam or hot water it needs and is invariably overheated while a room remote from the boiler is starved for the heat it needs even if its thermostat opens all the valves leading to that room.
Existing multi-zone systems, such as those used in school buildings therefore are predicated on having the steam or hot water supply available just outside the zone. But, a cold room cannot demand more performance than the custodian has programmed for the building average. These controls also involve complex pneumatic valve systems and long air runs between thermostat and radiator controls. A leak in the lines is a major problem.